Upon prescribing spectacle lenses to correct visual acuity, if the right and left eyes have the same visual acuity, the prescription is made by using lenses having the same refractive powers (dioptric powers) for both eyes. The right and left spectacle lenses thus prescribed are the same in curvature for a first refracting surface (the surface on an object side in a spectacles-wearing state, namely, a forward refracting surface) and a second refracting surface (the surface on an eye side in a spectacles-wearing state, namely, a rearward refracting surface) with each other, and since optical performance such as astigmatism, curvature of field, distortion aberration and the like is also the same as a matter of course, no problem based on the difference between right and left lenses occurs. It should be noted that refracting power (dioptric power) of spectacle lens is in general approximately the sum of refracting power of the first and second refracting surfaces and expressed by a unit called dioptric power (hereinafter referred to as D). The refracting power (surface power) of the first and second refracting surfaces is defined by curvature ρ of each surface (the unit is 1/m, and the radius of curvature R is 1/ρ) and a refractive index n of lens material as follows.surface power=(n−1)×ρ=(n−1)/R  (1)
In this case, the refracting power of the first refracting surface of a spectacle lens is called especially a base curve.
However, when prescriptions containing dioptric powers differ between the right and left eyes, either curvature of the first refracting surface or that of the second refracting surface, or both differ between the right and left eyes. Here, according to Tscherning's ellipse which is found by Tscherning as a mathematical solution to eliminate astigmatism of a spectacle lens, an optimum base curve (refracting power of the first refracting surface) to eliminate astigmatism differs according to dioptric power of lens. Therefore, according to the Tscherning's ellipse, when the visual acuity differs between the right and left eyes, base curves of right and left lenses need to be different. When a lens is designed according to the Tscherning's ellipse, if the difference in visual acuity between the right and left eyes is large, the right and left base curves differ remarkably. Even according to a Percival form proposed by Percival relating to the curvature of field of a spectacle lens, the same result is obtained. When seen by a person other than the person wearing the spectacles from outside, the surface shapes of the right and left spectacle lenses appear remarkably unbalanced. Therefore, although optical performance is good, appearance is very poor. Moreover, since base curves differ according to refracting power (dioptric power), it is disadvantageous even from the point of view of production costs.
Accordingly, from the view points to reduce process cost, to improve outward appearance of lens, and the like, base curves are made common within a prescribed dioptric power range. Especially, in the case of progressive power lenses, design of reference progressive refracting surface, in which distribution of astigmatism and distribution of curvature of field from distance vision to near vision via intermediate vision for each base curve take optimum arrangement according to dioptric power in the prescription, is practiced, the aforementioned optical factors excluding prescribed dioptric power need to be laterally equal. Therefore, it is absolutely necessary to adjust the base curves of right and left spectacle lenses the same. In such a case, according to the Tscherning's ellipse, since the larger the curvature of base curve to vision acuity, the more the astigmatism is improved in a practical range as a spectacle lens, usually, either of right and left spectacle lenses having larger curvature in base curve is used to the other one. Accordingly, when the right and left lenses contain plus dioptric power, base curve on a stronger side of the plus dioptric power is used, and when the right and left lenses both contain minus dioptric power, base curve on a weaker side of minus dioptric power is used. However, in the lenses produced in this manner, deviations from the optimum base curves corresponding to each vision acuity for the right and left eyes differ right and left, which results in unbalanced optical performance. As a method to solve such a disadvantage, ensure the optical performance within a permissible range, and make the outward appearance favorable, known is a method to put a difference in curvature of the first refracting surface between right and left lenses within a prescribed range after ensuring prescribed items, optical performance, and the like (refer to Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-open No. 2002-202482